1. Field of Invention
The present invention relates to a surface acoustic wave device that can be used for widespread application as a band-path filter in mobile communication devices, such as a cellular phone, and as a resonator of a reference clock, and so on, and a method of manufacturing the same.
2. Description of Related Art
A related art surface acoustic wave device has a structure, as shown in FIG. 9 for example, in which an IDT (Inter Digital Transducer) electrode 20 constituted by combining at least a couple of comb electrodes formed of a conductive film is provided on a piezoelectric elastic substrate (piezoelectric substrate) 10, while ladder reflecting electrodes 30 formed of a conductive film, likewise are provided at both sides of the IDT electrode 20.
When an electrical signal is applied to the IDT electrode 20, which is at the center, conversion from the electrical signal to a surface acoustic wave (hereinafter SAW) and reverse conversion thereof is implemented on a surface of the piezoelectric substrate 10. The reflecting electrodes 30 reflect a surface acoustic wave generated on a surface of the piezoelectric substrate 10 so as to cause resonation. By utilizing this, a resonator, a band-path filter, and so on can be constituted.
Examples of such a surface acoustic wave utilized in a surface acoustic wave device include a Rayleigh wave having major components that are displaced vertical to a surface of the substrate 10 and displaced along a surface wave propagation direction, and a leaky surface acoustic wave (Leaky Wave) which propagates while emitting energy to inside of the substrate. Moreover, the related art includes an SH (transverse wave) type surface acoustic wave.
This SH type surface acoustic wave is in a wave mode in which energy of a displacement component vertical to a surface wave propagation direction and that parallel to a surface of the substrate 10 is confined to a substrate surface by the IDT electrode 20 and so on. The SH type surface acoustic wave is essential to achieve higher frequency of a surface acoustic wave device since acoustic velocity (propagation velocity) thereof is larger than that of a Rayleigh wave and so on.
Specifically, since it has been disclosed in the related art that the center frequency f of the surface acoustic wave device satisfies relationship f=v/λ (here, v is propagation velocity of a surface acoustic wave, and λ is wavelength), in order to achieve higher frequency of the device, wavelength λ is decreased. Specifically the pitch between electrode fingers determining the length of wavelength λ is decreased. However, the decreasing of a pitch is limited because of the limit of a manufacturing device. Thus, utilizing a surface acoustic wave whose propagation velocity v is large enables the aim to be achieved effectively.
As such a SH type surface wave having large propagation velocity, for example, a +90° X-propagating transverse leaky wave of an ST-cut quartz substrate that is a rotated Y-cut plate shown in Japanese Unexamined Patent Publication No. 61-73409, and a BGS wave (Bleustein-Gulyaev-Shimizu wave) described in Kiyoshi Nakamura et al. “Research on a transverse (SH type) surface acoustic wave and application thereof to communication devices” (online) (searched on Aug. 18, 2003) internet <http://www.ecei.tohoku.ac.jp/˜nakamura/shsaw.html>, which is a pure transverse surface acoustic wave, and a Love wave, an STW (Surface Transverse Wave: propagation velocity 5100 m/s) described in Implementation Report on Assistance Project for Industry-University Cooperative Research of Japan Society for the Promotion of Science, “High Performance GHz Range Surface Transverse Wave Resonant Devices. Applications to Low Noise Microwave Oscillators and Communication System”, 1995, pp. 132-137, which is excited in a quartz substrate, and so on has been disclosed.
In the case of a device utilizing an SH type surface wave, whose propagation velocity is large, higher frequency can be achieved comparatively easily. However, it has a defect, such as low reliability against temperature since variation in frequency with respect to temperature is large compared to a device using an ST-cut Rayleigh wave as shown in FIG. 10.
Thus, a related art method is disclosed in Japanese Unexamined Patent Publication No. 2002-76835. In this method, with using 37°-45° rotated Y-cut quartz substrate in which temperature coefficient of frequency (TCF hereinafter) of an STW is plus, and a thin film whose TCF is minus, the thin film having thickness depending on the cut angle of the quartz substrate is formed on the quartz substrate so as to make TCF zero (cancel out TCF), and thereby enhancing frequency-temperature characteristics. Here, TCF is represented by the following formula and is the slope of frequency with respect to temperature around room temperature in general.TCF=f1(δf/δT)(ppm/degrees centigrade)
In addition, in Japanese Unexamined Patent Publication No. 10-224172, on a quartz substrate with Euler angle (0°, 123°-177°, 90°) in which temperature coefficient of delay (hereinafter TCD) of an SH type leaky wave has a minus value, a piezoelectric thin film whose TCD is plus is formed, so as to make TCD zero, and thereby enhancing frequency-temperature characteristics. Furthermore, the relationship between Euler angle and the thickness of a thin film (ZnO film) to increase electromechanical coupling coefficient k2 contributing to widening of band of a device is illustrated. TCD has a relationship TCD=−TCF since the phase velocity of a surface wave and the group velocity thereof are almost equal to each other.